Conventionally, a seat back frame structure of a seat for a vehicle which is capable of reducing an impact load acting on a passenger has been adopted. The Japanese Patent Laid-open Publication HEI11-32865, for instance, discloses one example of such a seat back frame structure.
This seat back frame structure for a vehicle comprises a seat back frame structure integrally formed into a substantially C-shape, a tubular body connecting right and left longitudinal frames of the seat back frame structure, and a flattened tubular portion which is provided on this tubular body and is flattened so as to be bent due to the impact load acting on a passenger upon a rear collision of the vehicle.
According to the above seat back frame structure, a closed cross-sectional structure (a box structure) is formed by connecting the right and left longitudinal frames in the widthwise direction through said tubular body and thus, a safety of a passenger upon the collision can be secured due to the fact that said flattened tubular portion functions as an energy-absorbing portion which substantially restricts a destruction caused by the bending of the longitudinal frames upon the collision.
However, said conventional seat back frame structure has caused the following technical problems.
Firstly, the strength or the rigidity of the seat back frame structure can be improved by the closed sectional structure (a box structure) of the seat back frame structure and the provision of the energy-absorbing portion, however, an increase in the weight of the seat back frame structure is caused. This is contrary to a recent movement of a decrease in the amount of carbon dioxide emission by a decrease in the amount of the fuel being used.
In this connection, in order to limit such an increase in the weight, an example of adopting a high strength steel member such as an ultra high tension steel plate is disclosed in Japanese Patent Laid-open Publication 2008-67722. More specifically, left and right side frames with predetermined shapes into which the ultra high tension steel plate is formed by a press working is adopted.
However, such a high strength member is so expensive that the seat for a vehicle is expensive, even if the increase of the weight can be restricted.
Secondly, the strength or the rigidity of the seat back frame structure can be improved by making the seat back frame structure closed cross-sectionally and providing the energy-absorbing portion, whereby the structural integrity of the seat for the vehicle can be maintained in a case where the impact load is loaded, however, the upper portion of the seat back on which a head rest is mounted can be bent so as to be inclined toward the rear side of the vehicle due to the impact load toward the rear side of the vehicle.
In addition, while the vehicle is normally driven, the upper portion of the seat back can be slightly deformed or can vibrate due to the fact that an accelerator pedal or a brake pedal is suddenly depressed, or that the vehicle passes on a road with irregularities, which can give a passenger uncomfortable feelings.
If the rigidity of the side frames of the seat back is heightened in order to prevent such a deformation of the upper portion of the seat back, the increase in the weight of the seat back itself can be caused to badly influence the fuel consumption.